Highly accessible, nanotube electrodes for large surface area contact applications

ABSTRACT

An highly porous electrically conducting film that includes a plurality of carbon nanotubes, nanowires or a combination of both. The highly porous electrically conducting film exhibits an electrical resistivity of less than 0.1 Ω·cm at 25 C and a density of between 0.05 and 0.70 g/cm 3 . The film can exhibit a density between 0.50 and 0.85 g/cm 3  and an electrical resistivity of less than 6×10 −3  Ω·cm at  25  C. Also included is a method of forming these highly porous electrically conducting films by forming a composite film using carbon nanotubes or nanowires and sacrificial nanoparticles or microparticles. At least a portion of the nanoparticles or microparticles are then removed from the composite film to form the highly porous electrically conducting film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 11/853,963, filed Sep.12, 2007, and claims priority to U.S. Provisional Application No.60/843,939, filed on Sep. 12, 2006, the disclosure of which is herebyincorporated by reference in its entirety, including all figures, tablesand drawings.

FIELD OF THE INVENTION

The invention relates to highly porous nanotube films, methods offorming such films, and applications for such films.

BACKGROUND OF THE INVENTION

In the majority of devices and applications requiring electricalcontact, the required contact occurs at an essentially planar (2D)interface between an electrode and the material being contacted. Whatappears planar at long length scales generally acquires some corrugationat small length scales. However this corrugation is generally a naturalconsequence of the materials rather than a feature specificallyengineered into the interface. However, numerous applications canbenefit from an electrical contact that is 3-dimensionally distributed.

Examples of applications that can benefit from 3-dimensionallydistributed contacts include electrodes for electrochemical reactionssuch as for the production of hydrogen from water and proton generationat the anode in hydrogen fuel cells. In such applications the increasedsurface area electrode provides an increase in electrochemicallygenerated product. For super-capacitors, the increased electrode surfacearea greatly increases the device capacitance. Other applications, suchas for solar cells or photodetectors, where light must be absorbedwithin a semiconducting junction region possessing a built-in potentialto drive the photo-generated electrons to the cathode, can similarlybenefit from the extended active area volume that a 3-dimensionallydistributed electrode can provide. For electroluminescent deviceapplications increased surface area electrical contact to the activematerial can provide increased current injection, with concomitantincreases in light generation.

Recently, films of single-wall carbon nanotubes (SWNTs), which areelectrically conducting have emerged as promising electrodes for a broadrange of applications. Such films can be fabricated by various methodsincluding a method described in published U.S. Application No.20040197546 (hereafter the '546 application) to a group of inventorsincluding one of the present Inventors. The '546 application isincorporated by reference into the present application in its entirety.Briefly, the method described in the '546 application comprisesfiltration of a surfactant suspension of SWNTs onto the surface of afiltration membrane possessing pores too small for the SWNTs to passthrough. The nanotubes accumulate at the surface of the membrane forminga film. Subsequent washing removes residual surfactant, while dryingconsolidates the nanotube film. Transfer of the film to a substrate ofchoice requires appropriate selection of the membrane media to permitits dissolution in a solvent that can be tolerated by the substrate towhich the transfer is made. Such transfer generally proceeds by adheringthe membrane-backed nanotube film to the substrate, followed bydissolution of the membrane in the chosen solvent.

SWNT films so fabricated possess a tortuous path, open porosity in whichthe pores between nanotubes are defined by the overlapping and crossingnanotube bundles. The nanotubes tend to be self-organized into bundles,each possessing a varying number of nanotubes across their widths from afew to hundreds of parallel nanotubes, approximately 3 to 20 nm indiameter, with a typical diameter of ˜10 nm. FIG. 1 shows a scannedatomic force microscopy (AFM) image of a typical 70 nm thick filmsurface (bundles diameters appear greater than ˜10 nm only because oftip-sample convolution). This open porosity has the potential to providea structure having some of the desired, high surface area,3-dimensionally distributed electrical contact with another material.

Examination of FIG. 1 suggests that voids between crossing nanotubebundles have dimensions of tens to hundreds of nanometers across.However, the inference of pore volumes from such surface images ismisleading. In the film formation process disclosed in the '546application the nanotube bundles are uniformly dispersed in the dilute,aqueous suspension. The first bundles to land on the flat filtrationmembrane surface are forced to lie essentially parallel to the surface.Because the film grows at a uniform rate (with nanotube bundles lyingacross those deposited before them), subsequently deposited bundles takeon the same planar orientations. The result is a film morphology whereinthe nanotubes have random in plane orientations, but lie in stackedplanes, with two-dimensional anisotropy similar to a biaxial orientedpolymer film. This would suggest that the average dimension of the poresbetween bundles, in the direction perpendicular to the filtrationmembrane surface (the thickness direction of the film), is that of onlya few nanotube bundle diameters. This analysis assumes however that thenanotube bundles are rigid rods.

The nanotube flexibility, and surface energy minimization by van derWaals contact causes them to maximize their contact, acting to furtherreduce these pore volumes. A quantitative measure of the availableporosity is given by a comparison of the theoretical density of ahexagonal close pack array of nanotubes (using a prototypical 1.356 nmdiameter (10, 10) nanotube) and the experimentally derived density of afiltration method formed SWNT film. The former is approximately 1.33g/cm³ while the latter has been measured to be about 0.71 g/cm³. Hencethe as-produced filtration method described in the '546 applicationproduces SWNT films that achieve nearly 53% of their theoretical maximumdensity. Since this porosity is generally uniformly distributedthroughout the film, the average pore volume is generally of a size thatis smaller even than an average bundle volume.

There may be utility to infiltrating the porosity of films producedusing the process disclosed in the '546 application with anelectro-active medium and using the nanotubes as electrodes. However,the small size of these pores limits the utility of this structure for3-dimensional distributed electrode applications. The limitationsassociated with the small pore size depends on the specific application,two exemplary limitations being as follows:

1. As electrochemical electrodes the small pores yield slow dynamics forpermeating chemical species into the volume of the films, against thecountercurrent of reaction products that must get out. This will limitthe production rate of the desired species.

2. As photovoltaic electrodes, which are infiltrated with asemiconductor that generates a built-in potential at thenanotube-semiconductor interface, wherever the nanotubes defining thepores lie within a Debye length proximity of each other, theirpotentials will screen each other, reducing the potential gradient.Since that potential gradient provides the electromotive force forcharge transport away from the interface, such screening will limit thephoto-current and therefore the power generated by the photovoltaicdevice.

Thus, a need exists for nanotube and/or nanowire films having higherlevels of porosity, significantly higher pore volumes, and a higherratio of surface area to film volume as compared to films produced bythe method described in the '546 application, or other methods ofnanotube film fabrication such as spray coating or Langmuir-Blodgettassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features andbenefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

FIG. 1 is a scanned AFM image of a 70 nm thick SWNT film based on themethod disclosed in the '546 application. The color-graded, verticalvariations in the scanned image occupy only a small potion, near themiddle, of the vertical scale (right).

FIGS. 2(A) and (B) are scanned AFM images of a polystyrene sphere/SWNTcomposite film according to the invention before and after dissolutionof the polystyrene spheres. All scales are the same as in FIG. 1. Notethe far greater vertical variations as compared to the scanned imagesshown in FIG. 1.

FIG. 3(A)-(C) are scanned tilted, AFM image surface plots of a standardSWNT film, a composite polystyrene sphere/SWNT film before spheredissolution according to the invention, and after sphere dissolution,respectively.

FIGS. 4(A) and (B) are scanned electron micrographs (SEMS) of acomposite nanosphere/SWNT film before and after nanosphere dissolution,respectively. High resolution AFM imaging shows that what appears inFIG. 4( b) to be a thin over coating is in fact poorly resolvednanotubes.

FIG. 5 compares the amount of charge stored (in μCoul) as a function oftime on two electrolytic capacitors, one comprised of two porous SWNTfilm electrodes and the other of two standard SWNT film electrodes. Thecurve shown is the instantaneous charge on the porous film devicedivided by the corresponding instantaneous charge on the standard filmdevice. The same mass of nanotubes per geometric surface area is exposedto the electrolyte (0.1 M KCl) in each device so that the onlydifference between the two devices is the morphology of the SWNTelectrode films (porous versus standard).

FIG. 6 shows the ratio of the charge on the porous film to the charge onthe standard film, as a function of time, during the 2 second dischargecycle, from the data in FIG. 5. The porous film device is seen to have acapacitance that exceeds that of the standard film device by 42%.

DETAILED DESCRIPTION OF THE INVENTION

One of the assumed advantages of nanotube films in electrodeapplications is their high surface area for electrical contact, derivingfrom their nanoscale widths. However, as recognized by the presentInventors, the nanotube films possess less porosity, and therefore lessaccessible surface area than might be anticipated. Based on thisunderstanding, methods are described herein for forming highly porousnanotube or nanowire films to increase the accessibility of the nanotubeor nanowire surface area and to thereby maximize the interfacial contactarea and volume between the nanotubes or nanowires and infiltratedmaterials.

A method for forming porous carbon nanotube or more generally nanowirefilms comprises the steps of forming a composite film comprising (i)carbon nanotubes or nanowires and (ii) sacrificial nanoparticles ormicroparticles, and removing at least a portion of the sacrificialnanoparticles or microparticles from the composite film to form a highlyporous nanotube or nanowire film. Films according to the inventionprovide enhanced pore volumes and high levels of accessible surface areawithin the body (volume) of nanotube or nanowire films. Resulting filmshave been found to provide both high porosity as well as high electricalconductivity.

In one embodiment, sacrificial particles of a uniform particle size(uniform defined as ±5% of a mean size) are utilized. In anotherembodiment, a variety of particle sizes is used, such as a distributionincluding both nanoparticles and microparticles.

As used herein, “nanoparticle” is used to refer to particles with atleast one axis less than 100 nanometers. As used herein, “microparticle”is used to refer to particles with at least one axis between 100nanometers and 100 micrometers. Although both nanoparticles ormicroparticles can generally be used with the invention, such particleswill be referred to herein as nanoparticles for convenience.

Nanoparticles useful in the present invention can have an axis between10 nanometers and 100 microns, inclusive. The nanoparticles useful inthe present invention can have a diameter between 10 nanometers and 100microns.

In one embodiment, the forming step can comprise codepositing the carbonnanotubes or nanowires and the sacrificial nanoparticles. In anotherembodiment, the forming step can comprise alternating deposition ofcarbon nanotubes or nanowires and deposition of the sacrificialnanoparticles.

In yet another embodiment, the forming step comprises a filtrationmethod based on the filtration method described in the '546 application.The filtration method comprises providing a porous membrane, dispersinga plurality of nanotubes or nanowires along with the sacrificialnanoparticles into a solution, the solution including at least onesurface stabilizing agent for preventing the nanotubes or nanowires andsacrificial nanoparticles from flocculating out of suspension, applyingthe solution to the membrane, and removing the solution, wherein thenanotubes or nanowires and sacrificial nanoparticles are forced onto asurface of the porous membrane to form the composite film on the surfaceof the membrane.

Using the filtration method the sacrificial nanoparticles and themembrane are chosen so that the sacrificial nanoparticles are too largeto penetrate through the filtration membrane, but small enough toco-deposit with the nanotubes during the film formation. The filtrationmethod film formation process can proceed with the nanoparticles in amanner that closely follows the corresponding process with the nanotubesas described in the '546 application.

Once the resulting film has been washed to remove the excess surfactantand is dried, it comprises a composite film of sacrificial nanoparticlesor microparticles randomly entrapped between the nanotubes comprisingthe nanotube film on the surface of the membrane. To generate theincreased void volume within the nanotube film a void formation step isutilized where the sacrificial nanoparticles are removed from the film.

The sacrificial nanoparticles can be removed after composite filmformation by a variety of methods including dissolution, evaporation,pyrolysis, oxidation or etching processes. In the case of the filtrationmethod, the removal process can occur before, during or after a transferof the film to a substrate.

In a first removal embodiment, referred to as dissolution, thesacrificial nanoparticles or microparticles selected for use are solublein the same solvent as the filtration membrane on which the film isformed. The sacrificial nanoparticles simultaneously dissolve during thedissolution of the membrane in the transfer of the film to thesubstrate.

In a second removal embodiment, referred to as etching, if thesacrificial nanoparticles selected for use are insoluble in the solventused to dissolve the membrane on which the film is formed, the particlecontaining film can be transferred to a substrate and the sacrificialnanoparticles subsequently dissolved, etched or vaporized away to yieldthe desired highly porous film.

In a third removal embodiment, if the membrane is not adversely affectedby the sacrificial nanoparticle removal method, the particles can firstbe dissolved, etched or vaporized away and subsequently the porous filmtransferred to a substrate followed by dissolution of the membrane.

The method can further comprise the step of doping the SWNT film asdescribed in the '546 application to provide either n-doped or p-dopedfilms. Dopants can be selected from halogens and alkali metals or morecomplex molecular species all of which can bond ionically to thenanotubes upon the charge transfer, or can be bonded by non-covalent pistacking interactions, along with a charge transfer, or finally cancovalently bond to the nanotubes, thereby effecting the charge transfer.

The electrical conductivity of the films depends on the degree ofporosity. Although not required to practice the present invention andnot wishing to be bound to this theory, Applicants provide themechanisms believed to be operable which explain the electricalproperties of films according to the invention. The main impedance tocurrent flow in a nanotube film occurs in charge transport from nanotubeto nanotube (the on tube resistance is so much smaller than tube-tube“contact resistance” that the former is essentially negligible).Moreover, the smaller the overlap between two nanotubes in the film thegreater the impedance to charge transport between them since “contactresistance” depends inversely on the area of the contact. Consequently,if two films of the same geometric area are made from the same quantity(mass) of nanotubes, wherein one film is a standard flat film asdescribed in the '546 application while the other is a porous film madeas described herein, then the porous film, in order to encompass thegreater volume of pores must itself encompass more volume. Since thequantity of nanotubes is the same, the only way this can occur is if thenanotubes in the porous film possess less overlap with each other thanexists in the standard flat film. The porous film will consequently havea higher sheet resistance. As shown in the Examples below however thechange in sheet resistance in going from the standard to the porousfilms is not increased to a degree that degrades their utility.

As described above, prior art films possess a morphology in which thenanotubes tend to lie parallel to the plane of the film (2-D ordering).The nanotubes in the porous films however possess a more 3 dimensionalmorphology in which many of the nanotubes have appreciable lengths thatare oriented perpendicular to the plane of the film. This isnecessitated by the fact that in the composite films, prior todissolution of the sacrificial nanoparticles, the nanotubes surround thesacrificial nanoparticles on all sides in random orientations, includingthose sides that lie perpendicular to the plane of the composite filmupon its formation. Once the composite film is formed the nanotubes lockeach other into position via van der Waals forces. When the sacrificialnanoparticles are removed (e.g. by dissolution) there is some relaxation(the degree depending upon the particle sizes), however because thenanotubes are stiff and locked together, the change in the 3-dimensionalfilm morphology can be small.

In one embodiment, the film consists essentially of (e.g. >95%) thenanotubes or nanowires. However, in other embodiments, films accordingto the invention can include mixtures of nanotubes and nanowires ormixtures of nanowires of distinct materials in any proportion desired.The films can also include in some fraction nanoparticles that are notsacrificial and participate in the functionality of the final porousfilms.

The porous films retain much of the optical transparency of standardnanotube films. The degree of transparency depends however on thesacrificial nanoparticle sizes used in formation of the films. For twofilms one standard and one porous that contain the same mass of nanotubematerial per geometric surface area the absorptive, nanotube material,path lengths through the film are the same (ignoring nanotubeorientation dependent complications) so that the amount of incidentlight absorbed in passing through each of the films is (to first order)the same. The light transmitted by the porous film will nevertheless belower due to scattering of incident light out of the forward directedbeam. In visual observation such scattering manifests as a haziness ofsome porous films. The scattering occurs because the film is comprisedof dissimilar materials (the nanotubes and the air filled voids)possessing distinct indexes of refraction. The degree of scatteringdepends on the size of the inhomogeneities in the refractive index,relative to the wavelength of the radiation. For ˜200 nm voids in aporous film the sizes of the inhomogeneities are themselves too small toresult in scattering of visible light, however, statistical variationsin the density of the 200 nm voids are themselves large enough to inducesome scattering and impart some haze to films made with 200 nmsacrificial nanoparticles. Depending on the application, such scatteringis not necessarily detrimental. In solar cell applications scattering oflight throughout the film is in fact beneficial because it providesadditional opportunities for light absorption. In applications wherehaze is undesirable the films will typically be infiltrated with amaterial other than air. The index of refraction of such material maynaturally lie closer to that of the nanotubes, or might be tailored todo so. Such index matching avoids the interfacial reflectionsresponsible for the scattering thereby avoiding any haze. Example ofthis is provided by the porous film (made using the 200 nm sacrificialparticles) immersed in methanol, which exhibits a clarityindistinguishable from standard films (i.e. no haze).

There is a broad array of possible distributed electrode applicationsthat can benefit from enhanced pore volumes and high levels ofaccessible surface area within the body of nanotube or nanowire filmsaccording to the present invention. For example, applications involvingelectrochemical reactions including their use in fuel cells can benefitfrom the invention. Also, applications involving charge storage, such ascapacitors and batteries can benefit from the invention. Moreover,applications involving charge injection and applications involving lightemission, such as photovoltaic conversion can benefit from theinvention. Finally, products that can benefit from the inventioninclude, but are not limited to, super-capacitor, battery, fuel cellelectrodes, solar cells, and solid state lighting.

EXAMPLES

It should be understood that the Examples described below are providedfor illustrative purposes only and do not in any way define the scope ofthe invention.

As a demonstration of the present invention using the filtration methodembodiment, nanoparticles comprises polystyrene nanospheres (uniformdiameters of about 200 nm) and filtration membranes of a mixed celluloseester (100 nm pores), both being soluble in acetone were utilized. Thequantity of nanospheres used was based on that estimated to formapproximately 3 monolayers of hexagonal close-packed (hcp) spheres (˜520nm thick without nanotubes). The quantity of nanotubes used was thatcalculated to form a film ˜80 nm thick in the absence of thenanospheres. FIG. 2( a) shows a scanned AFM image of the composite filmsurface prior to dissolution of the polystyrene spheres on the membranesurface (prior to film transfer). FIG. 2( b) shows a scanned image ofthe film after the transfer of the film to a smooth Mylar substrate,during which process the polystyrene spheres have dissolved. FIGS. 3(a)-(c) are scanned tilted AFM image surface plots of the films of FIGS.1, 2(a) and 2(b), respectively.

It is noted that following the dissolution of the nanospheres in thecomposite films, and their subsequent solvent washing to remove anyresidual polymer, the films were dried for the purpose of imaging.Because liquids exert surface tension forces as they dry, and theseforces tend to collapse flexible nanostructures, it is anticipated thatthe film porosity and accessible surface area before drying is evengreater than what was observed in the images taken. If maximum surfacearea of contact is required between the nanotubes and a second materialthat is to be infiltrated into the porous nanotube film, it is importantthat, where possible, such infiltration occur without drying of thenanotube film following the nanoparticle dissolution.

To measure differences in the sheet resistance between standard andporous films both a standard (flat) film and a porous film of equalgeometric areas were formed from the same mass of SWNT material. Theflat film thickness was approximately 80 nm and its sheet resistance(surface resistivity) was measured to be ˜75 Ω/square. It should benoted that nanotubes purified by nitric acid are doped by the acid to bep-type conductors, but also that the degree of doping can change withtime. The resistivity of the nanotubes therefore depends on theirpurification history. To ensure a fair comparison the flat and porousfilms were made at the same time, from the same batch of nanotubematerial. The porous film of this example was made using 200 nmpolystyrene spheres, which were dissolved away during the film transferto its substrate (Mylar for both films). Although the films containedthe same mass of nanotubes per geometric surface area the porous filmsheet resistance was measured to be 100 Ω/square. As anticipated, thisis larger than the sheet resistance of the standard film (75 Ω/square),the difference, however, is not very large considering that thethickness of the porous film was approximately 600 nm thick, nearly 8times greater than that of the flat film. Hence the porous films canretain the major fraction of the conductivity of the standard films.

As a quantitative measure of the enhanced accessible surface area in theporous films electrolytic capacitors were fabricated using two standardSWNT films as the electrodes in a standard film device and two porousfilms as the electrodes in a second porous film device. Each electrodeused the same mass of nanotube material per geometric surface area, andthe geometric area of each electrode exposed to the electrolyte (0.1 MKCl) was 0.866 cm². The standard film had a thickness of 80 nm. Theporous films were of the same type as described above, made with 200 nmpolystyrene spheres (which by themselves would have resulted in a hcpclose-pack thickness of 520 nm). The capacitors were each charged to 0.5V for 180 seconds. At the end of the 180 second period the potential wasinstantaneously switched (within 5 ms) to zero volts for two secondsafter which the potential was switched back to 0.5 V for 2 seconds. FIG.5 compares the amount of charge on each capacitor during the 2 seconddischarge and 2 second charge cycle. FIG. 6 plots the ratio of thecharge on the porous film to the charge on the standard film for thedischarge cycle. The ratio of 1.42 reached at ˜1.25 seconds shows thatthese porous films have 42% more accessible surface area than thestandard films. Because the porous and standard films expose the samemass of nanotubes to the same electrolyte over the same geometricsurface area, the measurement provides clear evidence that much of thesurface area in the standard films is not accessible and that this canbe greatly increased by the methods described here.

As additional examples, several additional sacrificial particle systemsand nanoparticle removal methods are described below:

1. Silica nanoparticles dissolved by HF.

2. Metal nanoparticles dissolved by acid, such as zinc nanoparticlesdissolved by HCl.

3. Depolymerization of polymeric particles using the ceiling temperatureeffect.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

1. An electrically conducting highly porous film, comprising: aplurality of carbon nanotubes, nanowires, or a combination of both, saidfilm providing an electrical resistivity of <0.1 Ω·cm at 25 C and adensity of between 0.05 and 0.70 g/cm³.
 2. The film of claim 1, whereinsaid nanotubes or nanowires comprises >95% single walled carbonnanotubes (SWNTs).
 3. The film of claim 1, wherein said film consistsessentially of said nanotubes.
 4. The film of claim 1, wherein saidnanotubes are n-doped or p-doped.
 5. The film of claim 1, wherein saidfilm has inhomogeneous voids.
 6. The film of claim 1, wherein said filmhas 3-dimensional morphology wherein a multiplicity of said nanotubeshas an appreciable portion of their length oriented perpendicular to thefilm's plane.